ggplot output graphs different size

I have an R program that I run for weekly analysis. On my team, I use a Mac connected to a monitor, a college uses an iMac desktop, and finally one person uses a windows laptop. When we all run the same R code we have some ggplot's that save in a source folder. However When I run the analysis the graphs come out one size different than others when they are running this. What could be the issue?

It might be possible that the default size depends on the screen resolution.
Please give more details:
How do you actually save the plots? Using the export-function in RStudio? With ggsave?
Have you considered to define/ fix the output size?